1. Field of the Invention
The present invention relates to a method for purifying crude tetrahydrofurans such as tetrahydrofuran or alkyl tetrahydrofurans. The purified tetrahydrofurans (hereinafter shortened to THF) obtained by this invention are suitable to use as starting materials in the preparation of a polyether polyols. Such polyether polyols are very important because these materials are used in the preparation of polyurethane or polyester for use in elastomers or spantex. Polyurethane resins obtained by the co-polymerization of alkyl THF and THF exhibit superior elastic properties, resistance to lower temperature and hydrolysis, and consequently, are industrially useful substances in the chemical industry.
2. Description of the Prior Art
Many methods have been proposed for the purification of THF. For example, through the use of bleaching earth, Japanese Patent Publication 61-54029 obtained THF having a low carbonyl, bromine and peroxide content. There is, however, neither a stated objective of purifying nor disclosure directed to any kind of impurity. The removal of another THF impurity, THF peroxide, has been described in Japanese Patent Laid Open Publication 54-88256, by treatment with an activated carbon layer.
Further, aromatic aldehydes are an impurity in crude THF. A method to obtain THF from which aldehydes, such as n-buthylaldehyde, are removed is disclosed in Japanese Patent Publication 52-29. It involves a distillation following treatment with a primary amine having a high boiling point and non-volatile acid. These latter disclosed methods may be characterized as targeting a specified impurity, however, the procedure by which THF before purifying is prepared and the usage of the purified THF are not disclosed. Still further, when a primary amine of a high boiling point and non volatile acid are used alone, obtaining a purified THF would not be expected. Indeed, as disclosed in the aforesaid Japanese patent publication, they must be used together in order.
Aldehydes are also an impurity in THF that must be removed. For example, a method to obtain THF having less than 50 ppm aldehyde impurity is disclosed in Japanese Patent Laid open publication 54-88256. It involves treatment with boron hydride and afterwards distillation. In this publication, it is disclosed that when poly-oxitetramethyleneglycol (hereinafter shortened to PTMG) is prepared using THF which contains aldehydes, PTMG becomes colored.
A reported purification method directed towards THF obtained by the Reppe method comprises a distillation, a contact hydration in the presence of Raney nickel and a subsequent distillation is described in Japanese Patent Laid open publication 57-28076. In this publication, substances are listed which cause discoloration of the glycol polymer produced from THF, however, the substance mainly responsible for discoloration is not disclosed. In addition, expensive materials must be used when purifying THF by this method.
THF purified by the method disclosed in Japanese Patent Publication 10-29280 is obtained in greater than 99.9% purity. However, the disclosed process is lengthy and uses expenses materials. Crude THF is contacted with water and/or acetic acid in the presence of a high acid cationic ion exchanging resin. The acetic is then removed by distillation. The THF is then hydrated in the presence of a precious metal catalyst, and then purified by further distillation. In this method, the impurities to be removed are limited to dihydrofuran and N-butylaldehyde, and it is reported that PTMG obtained from this high purity THF is not colored. However, this reference provides no clarification as to which compound is responsible for the discoloration.